US 6483881 B1 Abstract The present invention relates to a method of reducing complexity in a decoder which uses trellis decoding that can easily reduce the complexity of the decoder as much as required by reducing the paths itself using the statistics of path metric values and the reliability of the received signal. The method comprises first step of determining the reliability of bits received at the current level after initializing the level of the trellis and a flag, second step of determining the paths to be taken into account according to the reliability of received bits and computing and storing information about the determined paths and third step of obtaining the statistics of path metric values and a reference path metric by utilizing the determined path information at the current level, reducing the number of paths, and moving to the next level when total number of paths is larger than the number of maximum paths to be maintained.
Claims(7) 1. A method of reducing complexity which can be applied to a decoder using trellis decoding, comprising the steps of,
a) determining a reliability of bits received at a current level after initializing a level of trellis and a flag;
b) determining paths to be taken into account according to the reliability of received bits and computing and storing information about the determined paths; and
c) obtaining statistics of path metric values and a reference path metric by utilizing determined path information at the current level, reducing the number of paths, and moving to a next level when a total number of paths is larger than a maximum number of paths to be maintained.
2. The method according to
b1) checking if the flag is 1 or if the reliability of the received bits is the minimum necessary to determine the paths to be taken into account according to the reliability of the received bits;
b2) extending the paths to take into account all possible cases if the flag is 1 or the reliability of the received bits is the minimum necessary, and then computing and saving information for all paths;
b3) maintaining the most likely paths and removing other paths if the reliability of the received bits is larger than a minimum value and if the flag is 0, and then computing and saving information for the selected most likely paths;
b4) checking if the total number of paths at the current level is larger than the maximum number of paths to be maintained;
b5) determining if the total number of paths is larger than 0 if the total number of paths at the current level is found not to be larger than the maximum number of paths to be maintained in said step b4);
b6) setting the level to 0 and setting the flag to 1 if the total number of paths is found not to be larger than 0 in said step b5), and then checking a soft-decision information for the received signal at the current level to measure the reliability; and
b7) moving to the next level if the total number of paths is found to be larger than 0 in said step b5), and then checking the soft-decision information for the received signal at the next level to measure the reliability.
3. The method according to
c1) obtaining statistics of path metric values, wherein the statistics include a number of paths extended at the current level of the trellis, a mean and a standard deviation of path metric values for the case when the total number of paths at the current level is larger than the maximum number of paths to be maintained,
c2) obtaining the maximum number of paths to be maintained in the decoder using said statistics of path metric values and obtaining the ratio of the number of paths extended at the current level to the maximum number of paths to be maintained in the decoder, and obtaining a reference path metric; and
c3) moving to the next level after removing the paths having a larger path metric than the reference path metric.
4. The method according to
where, A is the maximum number of paths to be maintained at each level, S
_{i }is the total number of paths at the i^{th }level, m_{pmi }is the mean of path metric values at the i^{th }level, σ_{pmi }is the standard deviation of path metric values at the i^{th }level, and Pm_{ri }is the reference path metric at the i^{th }level.5. The method according to
6. The method according to
7. A computer readable recording medium storing instructions for implementing the method for reducing complexity which can be applied to a decoder using trellis decoding, the method comprising the steps of;
a) determining a reliability of bits received at a current level after initializing a level of trellis and a flag;
b) determining paths to be taken into account according to the reliability of received bits and computing and storing information about the determined paths; and
c) obtaining statistics of path metric values and a reference path metric by utilizing determined path information at the current level, reducing the number of paths, and moving to a next level when total number of paths is larger than-a maximum number of paths to be maintained.
Description 1. Field of the Invention The present invention relates to a method of reducing complexity in a trellis decoder which uses trellis decoding that can easily reduce the complexity of the decoder as much as required by reducing the paths itself using the statistics of path metric values and the reliability of the received signal. 2. Art Background In decoding schemes which use the trellis decoding such as Viterbi decoding, as the complexity of the trellis increases the performance of the decoder increases, but computational complexity of the decoder also increases. Many studies have revealed that in these schemes almost the same performance can be obtained as the maximum likelihood decoder even though less paths are taken into account than the number of paths which are required in the maximum likelihood decoder. In these schemes the best A paths are chosen out of total S paths. It takes lot of time, however, to carry out the algorithm of choosing A paths if S gets large. Thus, they may become less effective algorithm than the maximum likelihood decoding scheme. Prior schemes of reducing complexity are to select a part of paths that exist in the trellis of the code. Reliability of the received bits is usually used to select the part of paths. Alternatively they select arbitrary number of paths having a good path metric value. But no prominent method has been suggested for selecting the part of paths. Method of sorting values like path metric values has been generally used in the prior schemes. However, as the number of paths to be selected increases, it takes lot of time to carry out the sorting algorithm itself, so it is meaningless to reduce the complexity. Thus, in the trellis decoding algorithm new scheme is essentially required which can reduce the complexity of the decoder without increase of amount of computations and which can reduce decoding time by setting aside paths almost without chance of being selected, using the statistics of path metric values and the reliability of the received signal. It is thus an object of the present invention to provide a method of easily reducing, as much as required, the complexity of a decoder which utilizes trellis decoding without increasing the amount of computations. This is accomplished by reducing the number of paths using the reliability information of the received signal and the statistics (the mean and the standard deviation) of path metric values in a trellis decoder. The method according to the present invention to achieve the above-mentioned purpose can be applied to a decoder using trellis decoding for reducing complexity and comprises a first step of determining the reliability of bits received at the current level (time stage) after initializing the level of the trellis and a flag, a second step of determining the paths to be taken into account according to the reliability of received bits and computing and storing information about the determined paths, and a third step of obtaining the statistics of path metric value a reference path metric. The third step is performed by utilizing the determined path information at the current level, reducing the number of paths, and moving to the next level when the total number of paths is larger than the maximum number of paths to be maintained. The objects, features and advantages of the present invention will be apparent to one skilled in the art from the following detailed description in conjunction with the accompanying drawings, in which: FIG. 1 is an exemplary block diagram of a general convolutional encoder; FIG. 2 is a diagram for explaining the trellis structure of FIG. 1; FIG. 3 is a flow chart which shows entire flow of the complexity reduction method according to the present invention; FIG. 4 FIG. 5 is a detailed flow chart of an embodiment for the path reduction procedure using statistics of the path metric of FIG. FIG. 6 shows the bit-error rate performance when the complexity reduction method of an embodiment of the present invention is applied to the Viterbi decoding scheme of the (15,11) Reed-Solomon code; and FIG. 7 is a complexity comparison chart when the complexity reduction method of an embodiment of the present invention is applied to the Viterbi decoding scheme of the (15,11) Reed-Solomon code. Now, detailed description of the present invention follows referring to the attached figures. FIG. 1 is an exemplary block diagram of a general convolutional encoder in which Referring to FIG. 1, the convolutional encoder shown in the figure has a simple structure with the code rate being 1/2 and the constraint length being As shown in FIG. 1, the convolutional encoder outputs Since the structure and operation of the convolutional encoder with code rate 1/2 and constraint length In general, the number of trellis states is determined according to the number of memories The trellis diagram of the convolutional encoder is shown in FIG. State transitions at each state of the trellis shown in FIG. 2 is determined by the new information bits inputted to the memories Referring to FIG. 2, each node denoted by a black dot in the trellis diagram shows the state of the encoder at certain time. The line connecting a node and a node is called a branch, which denotes a transition from the state of the encoder at the current time point to the state of the following time point. Each time point of the trellis (that is, x axis) is called a level of the trellis and the line composed of branches connecting nodes is called a path. As a result, the paths denote code words. In the Viterbi decoding scheme, metrics for all paths (i.e., code words) existing in the trellis is computed for the inputted bits, and the path with the smallest metric value is determined as a code word. The metric value is Euclidean metric in this embodiment. FIG. 3 is a flow chart which shows the entire flow of the complexity reduction method according to the present invention. The present invention utilizes the fact that in a decoder which uses the trellis decoding scheme like Viterbi decoding, almost alike performance to the maximum likelihood decoder can be achieved with only arbitrary A paths which is less than the total number of paths S and the fact that the probability distribution of path metric values at each level of the trellis is an almost Gaussian distribution. Thus, to select good A paths out of S paths at each level of the entire trellis, the present invention reduces the number of paths based on mean and standard deviation of the path metric values at each level. Hence it can reduce the complexity of the decoder by reducing the number of paths without any additional large computations. For example, the present invention make it possible to utilize an additional coding gains due to soft-decision decoding of a trellis decoder with a large amount of computations. That is, it can reduce the complexity of a decoder using statistics of path metric values even for the Viterbi decoder for Reed-Solomon codes, the complexity thereof being extremely high, or for the Viterbi decoder for convolutional codes with a large constraint length, the complexity thereof being too high. As any other linear block code as well as convolutional code can be expressed with the trellis, the trellis decoding technique like Viterbi decoding can be applied to them. So the soft-decision decoding can easily be applied to them achieving additional coding gain. As shown in FIG. 3, the complexity reduction method of the present invention first initializes the level of the trellis and a flag (i.e., level=0, flag=0)(Step Then, as the reliability of the received signal can be determined based on the soft-decision information, it determines the reliability by checking the soft-decision information for the received signal at the current level (Step If the flag is 1 or the reliability of the received signal is the smallest value (Step If the reliability of the received signal is larger than the smallest value and if the flag is 0, the decoder only keeps the most likely paths and neglects the rest of them (Step Thus, if there is little noise in the received signal, the reliability of the received signal is high and the number of paths to be considered is very small. On the contrary, if there is lot of noise in the received signal, the reliability gets low making the number of paths to be considered is increased. If the number of paths which satisfy the current conditions gets 0 (i.e., flag=1) while decoding is continued this way, paths are extended to take into account all possible cases (Step Then, it analyzes if the total number of paths at the current level of the entire trellis is larger than the maximum number of paths A to be maintained (Step If the total number of paths is found not to be larger than the maximum number of paths A as a result of the analysis, it checks if the total number of paths at the current level of the entire trellis is larger than 0 (Step If the total number of paths is found not to be larger than 0 as a result of the checking, it makes the level to 0 and sets the flag to 1 (Step If the total number of paths exceeds 0, it moves to the next level (Step If the total number of paths is found to exceed the maximum number of paths A to be maintained as a result of the analysis, the method computes the statistics of path metric values and obtains the reference path metric, then it reduces the number of paths using them (Step Multiple paths always exists at each level of the trellis. The number of paths at a level generally increases as the error-correction capability of the code gets high. Also, as the number of paths increases the probability distribution of the path metrics at each level gets close to Gaussian. The distribution of the number of path metrics at an arbitrary level for various decoding schemes are shown in FIG. 4 FIG. 4 Referring to FIG. 4 where, A is the maximum number of paths to be maintained at each level, S The decoder simply computes the reference path metric Pmr and eliminates the paths having path metrics larger than the reference path metric to select the best A paths. For practical systems, it could be easily implemented by normalizing the integral value into a table with mean being 0 and standard deviation being 1. To help understanding, the above mentioned technique will be explained with an example of a code rate 1/2 convolutional code with constraint length There are 64 total paths at each level for the above coding scheme. Assuming that only 12 of these paths are always to be maintained, the number of paths starts with 2 and increases to 4, 8, and 16, thereby exceeding the required 12 paths. At this time, A/S Then, paths with path metrics values larger than Pm FIG. 5 is a detailed flow chart of an embodiment for the path reduction procedure using statistics of the path metric of FIG. As shown in FIG. 5, the reduction method of the number of paths (Step Then, it calculates the maximum number of paths A to be maintained in the decoder by using the statistics of path metrics and the ratio A/S Next, it calculates the reference path metric Pmr using Equation 1 (Step Let's take a look at the gain attainable when the present invention is applied with an example of Viterbi decoding for (15, 11) Reed-Solomon code. There are approximately 1.7×105 paths to be extended in average for an information bit. But only 1024 paths might be maintained at each level to obtain similar performance. In this case the number of paths that should be extended for an information bit is approximately 1600. If the method suggested here is applied then further complexity reduction can be achieved. FIG. Referring to FIG. 6, it can be seen that the bit error rate performance curves when the present invention is applied to the Viterbi decoding scheme for the (15, 11) Reed-Solomon code are similar for the case Referring to FIG. 7, it can be seen that similar number of paths It can be seen, therefore, that the path reduction procedure using the statistics of path metric values suggested by the present invention is very effective and accurate. Furthermore, in the case While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without departing from the spirit and scope of the invention. Patent Citations
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